Shock absorbers are used in conjunction with automotive suspension systems to absorb unwanted vibrations which occur during driving. To absorb the unwanted vibrations, shock absorbers are generally connected between the sprung portion (body) and the unsprung portion (suspension) of the automobile. A piston is located within a pressure tube of the shock absorber and the pressure tube is normally attached to the unsprung portion of the vehicle. The piston is normally attached to a piston rod which extends through the pressure tube to be connected to the sprung portion of the vehicle. The piston divides the pressure tube into an upper working chamber and a lower working chamber both of which are typically filled with a hydraulic liquid. Because the piston is able, through valving, to limit the flow of the hydraulic liquid between the upper and the lower working chambers when the shock absorber is compressed or extended, the shock absorber is able to produce a damping force which counteracts the vibration which would otherwise be transmitted from the unsprung portion of the vehicle to the sprung portion of the vehicle. In a dual tube shock absorber, a fluid reservoir or reserve chamber is defined between the pressure tube and a reserve tube. A base valve is located between the lower working chamber and the reserve chamber to also produce a damping force which counteracts the vibrations which would otherwise be transmitted from the unsprung portion to the sprung portion of the vehicle.
Shock absorbers filled with hydraulic liquid have met with continuous success throughout the automotive industry. While meeting with success in the automotive industry, hydraulic liquid filled shock absorbers are not without problems. One problem associated with the prior art hydraulic liquid shock absorbers is their lack of ability to change the damping characteristics in response to the frequency of vibration. In order to overcome this deficiency, complex systems have been developed to produce hydraulic liquid filled shock absorbers which are relatively soft for high frequency vibrations while being relatively stiff for low frequency vibrations. Other problems associated with the prior art hydraulic liquid filled shock absorbers include the variability in their damping forces due to the temperature changes of the hydraulic liquid. As the temperature of the hydraulic liquid changes, the viscosity of the liquid also changes which significantly affects the damping force characteristics of the liquid. In addition, any aeration of the hydraulic liquid during operation adversely affects the operation of the damper due to the introduction of a compressible gas into the non-compressible hydraulic liquid. Finally, the hydraulic liquid adds to the weight of the shock absorber as well as presenting environmental concerns regarding the use of the hydraulic liquid.
In an effort to overcome the problems with hydraulic liquid filled dampers, the industry has designed compressed gas, preferably air, dampers. The use of a gas and particularly air as a damping medium produces a frequency dependent damper or shock absorber which is less sensitive to temperature changes when compared to the hydraulic liquid dampers. These air dampers are not adversely affected by aeration over time, they are lower in weight and they are environmentally friendly due to the elimination of the hydraulic liquid.
The continued development of these gas or air dampers has been directed towards the tunability of these dampers. The gas or air damper is inherently frequency dependent due to the compressibility of the damping medium. The further development of these dampers has been directed towards varying the damping characteristics in relation to one or more vehicle parameters.